Which sentence is considered more correct:
Wenn ich mich früher vorbereitet hätte, würde ich jetzt solche Probleme nicht haben.
Wenn ich mich früher vorbereitet hätte, hätte ich jetzt keine solchen Probleme.
It is a matter of generations and time. The form würde + infinitive is more recent and was tought to me in the early 80ies as Faulheits-Konjunktiv (lazy conjunctive), by a teacher born in the late 30ies. At that time, probably nobody would have contradicted her.
But you find this "lazy" form today more frequently, if not in books then in newspapers at least. The english construct (If + past-tense, then … would + infinitive) is also disregarded from time to time, albeit to lesser extent, like in other languages (french Si + imparfait/plusqueparfait, conditionnel I/II). Probably common to all languages is that conditional phrases sound more complicated. Hence there is an obvious tendency to simplify them with increasing alphabetisation or spread.
The origin of würde in turn is the conjunctive of the verb werden (as the auxiliary of future tense in german), see:
Wenn morgen die Sonne scheint, dann werde ich spazieren gehen.
And if you cannot really believe it, because it is already raining today:
Wenn morgen die Sonne schiene, dann würde ich spazieren gehen.
Both are correct, neither is "more" correct. You can always substitute the würde-variant for Konjunktiv II, no matter if it's for "haben" or another verb.
However, this is more frequently done in spoken language, or when the Konjunktiv II form is either perceived as awkward in modern German (flösse, stürbe, ...), or is the same as the indicative form (besuchte).
So in written or literary language, your second example would be preferred.
I'd say that synthetic Konjunktiv II is more or less mandatory for highly productive Verbs, ie. all auxiliaries, including modal auxiliaries. Therefore the second sentence is more correct, even in spoken language, although the first one is not ungrammatical. It is just uncommon - just imagine you'd form Simple Past with "did" all the time - it would be correct, but awkward.
Apart from the auxiliaries, there are a few verbs where it is still common to use synthetic Konjunktiv II, such as ginge, näme, sähe.